This invention relates generally to mechanisms for vibration isolation and, more particularly, to structures that allow fine pointing to be commanded to optics while simultaneously isolating the optics from any vibration that is present in a base platform on which optics or other precision equipment is to be mounted. The need for this type of vibration isolation arises in numerous applications. Some of these applications involve equipment installed on the ground, and others involve equipment installed on ships, aircraft or spacecraft. A typical application is the mounting of a telescope or other precise optical sensor such as an interferometer. The optical instrument as a whole must be pointed precisely in a desired direction. The optics must also be isolated from moderate to high frequency base motions, which, if transmitted, could distort the relative positions of the optical elements, thus causing aberrations such as wavefront error or defocusing.
Another application of the invention is the isolation of precision manufacturing equipment from base motion over a wide frequency range, such as 0.01 Hz to 1,000 Hz. Isolation from extremely low vibration levels is required, for example, in the fabrication of semiconductor wafers on the ground, and for microgravity crystal growth experiments in space.
It is well known that passive vibration devices, such as springs with passive damping devices acting in parallel, cannot always provide satisfactory isolation at low frequencies, such as below 5 Hz. Another difficulty with passive isolators is that they have a natural sag in the presence of gravitational forces. For a passive isolator having a natural frequency f.sub.0, the static vertical sag is given by the formula .DELTA.=g/2.theta..sub.0).sup.2, where g is the acceleration due to gravity. For a 10 Hz passive isolator, the sag is approximately .DELTA.=2.5 mm, and it is feasible to build springs with sufficient stroke, linearity and strength to support the weight of the payload. However, at lower isolation frequencies, such as approximately 1 Hz, the static sag in the isolator mount under gravity loads will be 100 times as great, i.e. .DELTA.=25 cm. This presents a difficult mechanical design problem, which is sometimes solved through the addition of complicated and bulky pneumatic gravity offload systems.
Conventional six degree of freedom kinematic mounting configurations, sometimes known as "Stewart platforms," do not, in general, provide decoupling between translational and rotational motions of the base. In other words, the centers of force of the conventional Stewart platform are such that, should the base be translated laterally with respect to the payload, tipping moments, in addition to lateral forces, can be applied to the payload, with a resultant tendency toward pointing errors. The coupling between translational and rotational motion in the conventional kinematic mount also means that isolation frequencies for the three translational motions cannot be specified completely independently of the isolation frequencies for the three rotational motions. Thus, since practical considerations make it difficult to achieve a low isolation frequency for translational motions, the coupling between translation and rotation in conventional kinematic mounts also precludes the implementation of a low isolation frequency for rotational motions.
Another type of known isolation mechanism uses active rather than passive devices. Included in this general type is any device that senses deflection or acceleration of a platform to be isolated, and applies a force or commands a displacement to compensate for the sensed acceleration or deflection. Various types of deflection sensors and actuators, such as piezoelectric transducers, are well known, as are acceleration sensors and force actuators, such as magnetic voice coil motors. Active vibration isolation over a broad bandwidth of frequencies using this fully active compensation approach will, in general, require a control system with great accuracy, high gain and considerable speed. Moreover, such fully active isolation requires a relatively high power consumption. Consequently, there have been attempts to combine the simplicity of passive isolation at higher frequencies with active vibration and position control at lower frequencies, to obtain isolation and accurate positioning over a broad bandwidth. Such combinations, prior to the present invention, have not been successful.